Many anti-tumor compounds are restricted in their use because of their narrow therapeutic index, that is, the toxicities induced when the compounds are administered above certain dose levels outweigh the benefits thereby afforded. Anthracycline (e.g. doxorubicin) therapy, for example, is limited in that administration of the drug at levels in excess of cumulative 500 to 550 mg doxorubicin/m2 produces a substantial risk of cardiotoxicity and myelosuppression (von Hoff, et al.). However, compounds such as doxorubicin often remain the drug of choice for particular forms of chemotherapy; therefore it would be quite useful to develop means of lowering the compounds' toxicities whilst maintaining their therapeutic potential.
One means of approaching this objective that has been tried for several decades is the design of prodrug molecules that are differentially activated in tumor tissue, that is, drug molecules inactive or significantly less active upon administration that are selectively processed in tumor tissue so as to be therapeutically active therein. Leu-Dox (the amino acid leucine conjugated to the anthracycline doxorubicin), for example, is a prodrug found to require hydrolysis of the amino acid from the prodrug by intracellular proteases in order to release the anthracycline (Boven, et al. (1990)). Conversion of Leu-Dox to Dox in mice occurs rapidly, although incompletely, to approximately 20% overall conversion (de Jong, et al. (1992 a)). A similar observation has been made upon administration of Leu-Dox to humans (de Jong, et al. (1992 b); Canal, et al.); in a Phase I trial, approximately 25% conversion of Leu-Dox to Dox occurred rapidly in the tumor tissue. Moreover, in a human ovarian tumor xenograft mouse model, Leu-Dox has been shown to be a more effective anti-tumor agent than free doxorubicin, at equitoxic doses (Boven et al. (1992)).
Conjugation of additional amino acids to Leu-Dox may further decrease the availability of this compound to cells which do not secrete the requisite protease, and hence, further limits the compound's activity outside of tumors. In this regard, for example, Denmeade et al. have shown that a peptide-doxorubicin pro-drug targeted to the prostate-specific antigen (“PSA”). Ac-HSSKLQ-Leu-Dox (SEQ ID NO: 211) is a substrate for the PSA protease and is active against prostate tumor cells which express the protease activity. Furthermore, other mono and dipeptide conjugates on anthracyclines in addition to Leu-Dox have also been shown to have biological activity (Masquelier, et al.; Baurain, et al.). While a comprehensive analysis of dipeptide-anthracycline conjugates has not been reported, compounds consisting of Leu-Leu-Daunorubicin, Ala-Leu-Daunorubicin, and Leu-Ala-Daunorubicin have been shown to have considerable biological activity.
Various matrix-metalloproteinases (“MMPs”) have been described, and have had associated with them identifiable peptide cleavage sites (Nagase, et al.; McGeehan, et. al.). Moreover, the association between metastatic tumor progression has been made. In this regard, multiple researchers have shown that the enzymes MMP-2, MMP-9 and, more recently, MMP-14 (MT1-MMP) are associated with tumor progression (see, e.g., McDonnell and Fingleton; MacDougall and Matrisian). Increased expression of MMP-2 has also been reported in lung, stomach and breast carcinomas as compared to corresponding normal tissues. Increased expression of MMPs is not limited to the tumor itself. Increased expression of MMP-2 and MMP-14 has been observed in stromal and endothelial cells which are proximal to the tumor (e.g., Soini, Brummer). Thus, the level of MMP expressed is elevated at the tumor site.
Elevated expression of MMPs in tumor and supporting tissues implies that elevated activity is also present. While pro-forms of MMP-2 and MMP-9 enzyme are secreted by cells and readily detected in human serum and urine (Garbisa, et al.; Moses, et al.), the active form of the enzyme is found on the cell surface. In the case of MMP-2, the pro-form can be activated at the cell surface by the transmembrane enzyme, MMP-14 (Sato, et al.; Kurschatt, et al.). Activation of pro-MMP-2 has also been described to occur through binding of the preform of the enzyme to an integrin (Brooks, et al.). Activation of MMP-9 has been shown to occur through specific binding to the cell surface antigen, CD-44 (Yu and Stamenkovic). Based on these findings, it is anticipated that elevated MMP protease activity will be highest on the surface of tumor cells, so differential activation of the pro-drugs will be highest at the tumor site.
Safavy et al. (A. Safavy et al. (J. Med. Chem. 42:49194924 (1999)) describe the attachment of a seven amino acid synthetic peptide to the antitumor agent paclitaxel.
Trouet and Baurain describe tumor-activated prodrug compounds in U.S. Pat. No. 5,962,216, issued Oct. 5, 1999.
WO 99/02175, WO 98/18493 and WO 98/10651 conjugate certain prostate specific antigen (“PSA”) cleavable peptides to cytotoxic agents.
WO 98/16240 attaches peptides to lipids, for subsequent inclusion of the resulting conjugates in liposomes so as to target delivery of the vesicles' cytotoxic agent contents to tumors.
WO 00/33888 describes peptide conjugates of doxorubicin that are processed by an enzyme called trouase.
WO 00/21571 describes the use of FAP (Fibroblast Activation Protein) to deliver doxorubicin to tumors.
WO 00/64486 claims MMP activated conjugates for delivery of substances to tumors.
However, there remains a need to develop chemotherapeutic prodrug compounds which are inactive or significantly less active upon administration, thereby lowering the compounds' toxicities, that are selectively processed in or near tumor tissue so as to become therapeutically active anticancer agents.
The current invention discloses novel compounds useful for the treatment of cancer which comprises a matrix metalloproteinase (MMP) enzyme-cleavable peptide conjugated to doxorubicin. Furthermore, the current invention discloses novel compounds useful for the treatment of cancer which upon cleavage by a matrix metalloproteinase produces a second peptide doxorubicin substrate which can be further cleaved or processed by aminopeptidases expressed in the tumor environment. None of the references above suggest the compounds of the current invention.